1. Field of the Invention
The present invention relates to a process and apparatus for the hot dispersing of paper fiber stock, which includes providing an aqueous paper fiber stock, pressing out part of the water in a thickening device to form a highly consistent paper fiber stock, heating the highly consistent paper fiber stock, and dispersing the heated highly consistent paper fiber stock with a disperser.
2. Discussion of Background Information
Processes of the above-mentioned type are used, e.g., to improve the quality of fiber stock that is obtained from recovered paper. It is known that paper fiber stock can be homogenized and thus substantially improved by dispersion. To that end, a highly consistent fiber stock is produced, which has a dry matter content of between 15 and 35%. Dispersers that are used for such processes are mostly very compact and have an excellent dispersing effect with high energy density. However, they require an even stock stream.
By thickening to 15 to 35%, a considerable part of the water previously present in the fiber stock is pressed out, whereby its viscosity during the dispersion substantially increases, and, if necessary, less water has to be heated with it. A particularly cost-effective machine for thickening is the screw extruder. With a screw extruder the fiber stock suspension is pressed out between a screw conveyor and a perforated jacket surrounding it, whereby the water escapes through the holes in the jacket. The highly consistent fiber stock thus obtained is pressed out of the screw.
The hot dispersing is carried out at a temperature that is far above the ambient temperature. Heating the fiber stock is usually carried out by the direct addition of steam, for which, e.g., a heating screw is suitable: a screw conveyor rotates in a horizontal housing, which screw conveyor loosens the fiber stock and brings it into contact with steam that is blown in. The stock heats up through condensation. For economic reasons this heating is carried out on the already thickened stock.
Modern installations for carrying out such a process should work automatically. They therefore feature one or more automatic control loops in order to render possible a sufficiently constant operation without constant human intervention. Particularly when recovered paper is used as raw material for producing the paper fiber stock, it is inevitable that the raw material properties change, in particular the composition of the fibers contained in the paper fiber stock and the amount of the non-fiber components. The aim should therefore be to design the regulation of an installation used for dispersing so that, despite the fluctuations mentioned, a paper fiber stock processed as evenly as possible can be supplied for further use.
A dispersing process is known from DE 37 28 890 with which the specific work of the disperser is regulated in that more or less dilution water is added before the dispersion as required. The process is effective, but not always sufficient and requires an increased thickening effort.